Usually water slurry methods have been used for producing explosive molding powders showing sensitive or insensitive characteristics depending on the type of binder employed in regard to the requirements of STANAG 4170 or MIL Std 2105 b. The testing procedures for sufficient insensitivity of explosives to meet “Insensitive Munition” status are described, for example, in U.S. Pat. No. 5,547,526.
The insensitivity of an explosive pellet depends on the quality of the high explosive (HE)-crystals, binder type and weight percent present in the pellet, densities close to 100% t.m.d. and the perfect coating on each crystal.
In U.S. Pat. Nos. 5,067,996 and 5,547,526, the influence of the flexibility of the binder on the insensitivity is shown. However, neither the castable HMX mixture of the '996 patent with 15 wt. %-18 wt. % binder, nor the pressable HMX mixture of the '526 patent with 5 wt. % binder fulfill today's requirement of non-rupture (i.e., maximum sustained burning of the explosive) of the encasement described in the '526 patent during fast cook off.
Only the U.S. Navy qualified PBXN 9 explosive molding powder described in U.S. Pat. No. 6,485,587 with a binder content of about 8 wt. % meets the insensitive requirements. The PBXN 7 mixture of the '587 patent is not considered because of its very high content of already insensitive, but low energy, 1,3,5-triamino-2,4,6-trinitrobenzene (TATB). The other explosive mixtures provided in the '587 patent are still in development as indicated by PBX “W”. The binder employed in the '587 patent consists of HYTEMP®, a polyacrylic elastomer, and di(2-ethylhexyl)-adipate (DOA).
Achievement of a very high t.m.d at minimized pressing forces, which provides explosive pellets that have minimized porosities as well as being substantially crack free, require the use of harmonic crystal classes having mean diameters in the ratio of 1:1/7:1/49:1/343 (approximately, for bimodal use, the first two ratios are used, while for trimodal use, the first three ratios are employed and for tetramodal use, all four ratios are used) and crystal class mass ratios of 70:30 at bimodal, 65:28:7 at trimodal, and 64:27:7:3 at tetramodal (these are rough values, depending on the crystal batches used).
The modality influences the characteristics of the explosive pellets and offers proper choice needed for specific munition types. Grain classes and weight ratios disclosed in the '587 patent, for example, are driven by the total crystal surface depended behavior of the binder system HYTEMP® 4454, a polyacrylic elastomer, and DOA and finally require, relative to the present invention, 50% to 100% higher pressing forces to approach a necessary high t.m.d. for an insensitive explosive and usually fail to achieve this insensitivity goal at binder contents lower than 6 wt. %.
Indicated by the modalities above, crystal class C with a mean diameter are usually chosen as coarse grains; however, there is a need to consider the significantly increased possibility of internal crystal failures, e.g., hot spots of large crystals, due to today's crystallization methods.
Another insensitivity influencing point for consideration is that if water is present or it elevated temperatures are used, the contents of the pellets tend to vaporize. A water content of 0.01%, for example, potentially produces a waterdamp volume of approximately 30% of the pellet volume, thus increasing small pores to critical hot spot pores at munition cook off.